The present invention relates to an electrostatic chuck for attracting and holding substrates such as semiconductor wafers, and especially relates to an electrostatic chuck that realizes quick attraction and separation of the substrate (wafer) without causing wafer vibration.
Heretofore, an apparatus that attracts and holds a wafer using electrostatic force, so-called an electrostatic chuck, is utilized to hold substrates such as semiconductor wafers in a semiconductor fabrication system and the like. There are two types of electrostatic chucks. One is a so-called monopole-type electrostatic chuck comprising a single electrode, wherein a certain amount of voltage as opposed to ground potential is applied to the electrode while the substrate is grounded through plasma etc., thus utilizing the electrostatic force generated between the wafer and the electrode. Another is a so-called dipole-type electrostatic chuck comprising either a pair of or plural pairs of electrodes, wherein voltage is applied between the electrodes by which reverse polarity charge is induced in the substrate, thus utilizing the electrostatic force generated between the substrate and the electrodes.
The voltage applied to the above-mentioned single electrode or a pair of (pairs of) electrodes can either be DC voltage or single-phase alternating voltage.
The prior art technology related to these types of electrostatic chucks are disclosed for example in Japanese Patent Laid-Open Provisional Publication Nos. 7-273177 (273177/95) 9-213780 (213780/97) and 6-244271 (244271/94).
However, the conventional electrostatic chucks have the following problems. First, according to the system where DC voltage is applied to the electrode(s), electric charge is accumulated in the back surface of the substrate while the wafer is being attracted in both the monopole type and dipole type apparatuses, and the accumulated charge makes the separation of the substrate difficult. Especially in a plasma processing unit and the like where the substrate is in contact with plasma, the substrate becomes negatively charged as opposed to the ground potential through plasma, so the amount of accumulated charge is varied according to the state of plasma, and thus the process of charge being accumulated in the back surface of the substrate becomes more complex. For example, Japanese Patent Laid-Open Provisional Publication Nos. 7-273177 and 9-213780 disclose a method of removing the charge being accumulated in the substrate by applying a reverse voltage so-called a charge eliminator thereto. However, this charge elimination process required in the conventional techniques deteriorates the throughput of the apparatus.
In comparison thereto, Japanese Patent Laid-Open Provisional Publication No. 6-244271 discloses a method of applying a single-phase alternating voltage according to which no electric charge is accumulated in the back surface of the substrate, so there is no need to perform the charge elimination process when separating the substrate. However, this prior art method has a drawback in that attraction force becomes zero when the voltage reaches zero, and as a result, the wafer starts to vibrate.
The object of the present invention is to provide an electrostatic chuck that does not require a charge elimination process when separating the substrate, which has been required in the conventional art where DC voltage is applied, and a method of treating a substrate using this electrostatic chuck. The present electrostatic chuck solves the problem of substrate vibration, which had been observed in the conventional apparatuses utilizing single-phase alternating voltage.
In order to solve the problems of the prior art, the present invention provides an electrostatic chuck characterized in applying an n-phase alternating voltage, wherein n is a number equal to or greater than 2. FIG. 1 illustrates a basic structure of the present invention, in which three-phase alternating voltage is applied (n=3). A substrate 5 is mounted on a sample stage 9 comprising an R-phase electrode 1, an S-phase electrode 2, a T-phase electrode 3 and an insulator 4, having a three-phase alternator 6 connected to the electrodes 1, 2 and 3 via switch 11. By switching the switch 11 on and off, the wafer 5 can be attracted to or separated from the stage.
The time variation of voltage applied between the substrate 5 and the R-phase electrode 1, S-phase electrode 2 and T-phase electrode 3 is shown in FIG. 3. Moreover, the time variation of attraction force provided between the wafer 5 and sample stage 9 is illustrated in FIG. 4. As can be seen from FIG. 4, the attraction force provided between the wafer 5 and sample stage 9 is not varied by time, and as a result, the wafer will not vibrate.
Moreover, since no DC component is included in the voltage applied to electrodes 1, 2 and 3, no charge is accumulated in the back surface of the wafer, which had caused problems when DC voltage was applied to the electrodes. Furthermore, even in a plasma processing equipment and the like where the substrate comes into contact with plasma and the substrate takes a negative potential as opposed to ground potential, by insulating the primary side and the secondary side of the three-phase alternator 6 and by maintaining the secondary side providing three-phase alternating voltage to the electrodes 1, 2 and 3 to a floating potential as opposed to ground potential, no DC voltage is generated between the wafer 5 and electrodes 1, 2 and 3, and thus no charge accumulates in the back surface of the substrate. According to the present invention, prompt attraction and separation of the wafer is realized simply by switching the switch 11 on and off, without having to perform a charge elimination process.
In other words, by utilizing three-phase alternating voltage, the present invention provides an electrostatic chuck that realizes smooth and secure attraction and separation of the substrate, that does not require a charge elimination process and that solves the problem of substrate vibration.
Such advantages of the present invention can also be enjoyed by applying an n-phase alternating voltage wherein n equals 4 or more. FIG. 2 illustrates the basic structure of the present invention in which a four-phase alternating voltage is applied (n=4).
Furthermore, the above advantages can be enjoyed by applying a two-phase alternating voltage (n=2). FIG. 15 illustrates the basic structure of the present invention in which a two-phase alternating voltage is applied (n=2). FIG. 16 illustrates the time variation of the R-phase voltage and S-phase voltage of a two-phase alternator 19. The phases of the R-phase voltage and S-phase voltage are varied by 90 degrees so that the attraction force applied to the substrate 5 and the sample stage 9 does not fall to zero, and as a result, the substrate will not vibrate.